Fibrous nonwoven web surge layer for personal care absorbent articles and the like

ABSTRACT

Disclosed herein is a lofty fibrous nonwoven web which is particularly well-suited for use as a surge layer in personal care absorbent articles including, but not limited to, diapers, training pants, incontinence garments, sanitary napkins, bandages and the like. The fibrous nonwoven web is made from a plurality of fibers heat bonded to one another to form a lofty nonwoven web having a basis weight of at least 20 grams per square meter, a void volume of between about 80 and about 117 centimeters per gram of web while under a pressure of 689 dynes per square centimeter, a permeability of about 8,000 to about 15,000 darcy, a porosity of about 98.6 percent to about 99.4 percent and a surface area per void of about 10 to about 25 square centimeters per cubic centimeter. The fibrous nonwoven web will have a saturation capacity of between about 55 and about 80 grams of 0.9 percent saline solution per gram of web and a compression resilience in both the wet and dry state of at least about 60 percent.

This application is a continuation of application Ser. No. 08/206,986entitled "Fibrous Nonwoven Web Surge Layer for Personal Care AbsorbentArticles and the Like" and filed in the U.S. Patent and Trademark Officeon Mar. 4, 1994, now abandoned.

FIELD OF THE INVENTION

The present invention is related to fibrous nonwoven webs which areuseful in, among other things, personal care absorbent articles orproducts. More specifically, the present invention relates to a loftyfibrous nonwoven web which due to its unique design parameters isdesigned for rapid intake, retention and subsequent distribution of bodyexudates into the absorbent portion of the personal care article.

BACKGROUND OF THE INVENTION

Desired performance objectives for personal care absorbent productsinclude low leakage from the product and a dry feel to the wearer.However, absorbent articles commonly fail before the total absorbentcapacity of the product has been utilized. Absorbent garments, such asincontinence garments and disposable diapers, often leak at the legs andwaist. The leakage can be due to a variety of shortcomings in theproduct, one being insufficient fluid uptake by the absorbent system,especially on the second or third liquid insults.

It had been found that urination can occur at rates as high as 15 to 20milliliters per second and velocities as high as 280 centimeters persecond. Conventional diaper absorbent structures, such as thosecomprising admixtures of absorbent gelling particles and cellulosicfluff pulp, may initially uptake fluids at rates of only 8 millilitersper second or less, based upon the web density and the concentration ofthe gelling particles. In addition, the initial uptake rates forconventional absorbent structures can deteriorate once they have alreadyreceived liquid surges into their structures. The disparity betweenliquid delivery and uptake rates can result in excessive pooling on thesurface of the fabric before the liquid is taken-up by the absorbentcore. During this time, pooled liquid can leak from the leg openings ofthe diaper and soil the outer clothing and bedding of the wearer.Attempts to alleviate leakage have included providing physical barrierswith such design features as elastic leg gathers and changing the amountand/or configuration of the absorbent material in the zone of thestructure into which the liquid surges typically occur. Absorbentgelling particles have also been included to increase the liquid holdingcapacity in various regions of the absorbent structure, however, suchabsorbent gelling particles do not have the rapid uptake rates ofconventional materials such as wood pulp and fluff which are alsocommonly used in absorbent cores. As a result, as the amount ofabsorbent gelling particles in the absorbent core structures areincreased in modern day diapers, oftentimes the uptake rate will tend todecrease.

Nonwoven materials such as carded webs and spunbonded webs have beenused as the body side liners in absorbent products. Specifically, veryopen, porous liner structures have been employed to allow liquid torapidly pass through them and to help keep the body skin separated fromthe wetted absorbent pad underneath the liner. In addition, other layersof material, such as those constructed with thick, lofty fabricstructures, have been interposed between the liner and absorbent pad forthe purpose of reducing wet back.

With conventional fluff-based absorbent structures, the cellulosicfibers when wetted can lose resiliency and therefore collapse. As aresult, the liquid uptake rate of the wetted structures may become toolow to adequately accommodate subsequent liquid surges. Where absorbentgelling particles are incorporated between the fibers to hold themapart, the gelling particles swell and do not release the absorbedfluid. Swelling of the particles can then diminish the void volume ofthe absorbent structure and reduce the ability of the structure torapidly uptake liquid.

The addition of more absorbent materials, such as secondary fluffpledgets, or absorbent gelling particles, have been employed to increaseholding capacity. The desired rate of liquid intake within sucharrangements, however, may not be sufficiently sustained duringsuccessive liquid surges.

Despite the development of absorbent structures of the types surveyedabove, there remains a need for improved absorbent structures which canadequately reduce the incidents of leakage from absorbent products suchas disposable diapers. There is therefore a need for a material andresultant product which can provide effective handling of liquid surgesand which can more effectively uptake and retain repeated loadings ofliquid during use.

SUMMARY OF THE INVENTION

Personal care absorbent articles such as diapers, training pants,incontinence garments and sanitary napkins are often required to acceptquick, large insults of body exudates which are beyond the short termabsorptive capacity of the product. As a result, it has been foundadvantageous to use surge layers within such personal care absorbentarticles.

Personal care absorbent articles generally have a fluid permeable bodyside liner and a liquid impermeable backing layer with an absorbent coredisposed therebetween. The material of the present invention is used asa surge layer disposed between the body side liner and the absorbentcore. In addition, it is helpful if the surge layer of the presentinvention is attached to the liner and the absorbent core to promoteliquid transfer.

To perform in the desired manner, the fibrous nonwoven web should bemade from or include a plurality of thermoplastic fibers heat bonded toone another to form a lofty nonwoven web having a basis weight of atleast 20 grams per square meter. In more refined embodiments the basisweight will range from about 40 to about 68 grams per square meter. Theweb can be made entirely from bicomponent fibers which are typicallycrimped and which will generally have a fiber denier equal to or greaterthan 2. Alternatively, the web can be made from a combination of fiberssuch as bicomponent fibers and polyester fibers. In such embodiments,the web will usually include at least 50 percent by weight bicomponentfibers. The resultant web will have a void volume of between about 80and about 117 cubic centimeters per gram of web at 689 dynes per squarecentimeter pressure, a permeability of about 8,000 to about 15,000darcy, a porosity of about 98.6 to about 99.4 percent, a surface areaper void volume of about 10 to about 25 square centimeters per cubiccentimeter, a saturation capacity between about 55 and about 80 grams of0.9 percent saline solution per gram of web and a compression resiliencein both the wet and dry state of at least about 60 percent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting fluid pressure versus time. The graph isused in conjunction with the calculation of permeability values formaterials according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a lofty fibrous nonwoven web whichis particularly well-suited for use as a surge layer in personal careabsorbent articles including, but not limited to, diapers, trainingpants, incontinence garments, sanitary napkins, bandages and the like.

Personal care absorbent articles typically include a liquid permeablebody side liner and a liquid impermeable backing layer or baffle with anabsorbent core disposed therebetween. As discussed in the background, acommon problem with many of these products and their designs is the factthat they will not accept rapid and/or multiple insults of body fluidsor exudates such as menses and urine in a sufficiently short period oftime without leaking. In an attempt to overcome this problem, manyproduct designs have included some sort of additional layer between thebody side liner and the absorbent core to act as a dash pot of sorts totemporarily absorb, hold and then discharge the particular body exudatetaken in from the liner. The present invention relates to a loftyfibrous nonwoven web which has been specifically designed and, whichwhen incorporated into a personal care absorbent article or product,provides an effective means for temporarily storing and thendistributing body exudates. This material is referred to as a surgelayer.

The fibrous nonwoven web of the present invention is used as a surgelayer disposed between the body side liner and the absorbent core. Thesurge layer is most typically placed between and in contact with thebody side liner and the absorbent core though other additional layersmay be incorporated into the overall product design if so desired. Tofurther enhance fluid transfer, it is desirable that the fibrousnonwoven web surge layer be attached to the layers directly above andbelow its exterior surfaces. To this end, suitable attachment meansinclude, but are not limited to, adhesives (water-based, solvent-basedand thermally activated adhesives), thermo bonding, ultrasonic bonding,needling and pin aperturing as well as combinations of the foregoing orother appropriate attachment means.

The processes used to form the fibrous nonwoven web include those whichwill result in a material which, as further described below, is verylofty and open in nature. Suitable processes include, but are notlimited to, airlaying, spunbonding and bonded carded web formationprocesses. Spunbond nonwoven webs are made from fibers which are formedby extruding a molten thermoplastic material as filaments from aplurality of fine capillaries in a spinneret with the diameter of theextruded filaments then being rapidly reduced, for example, bynon-eductive or eductive fluid-drawing or other well known spunbondingmechanisms. The production of spunbonded nonwoven webs is illustrated inpatents such as Appel, et al., U.S. Pat. No. 4,340,563, Dorschner etal., U.S. Pat. No. 3,692,618; Kinney, U.S. Pat. Nos. 3,338,992 and3,341,394; Levy, U.S. Pat. No. 3,276,944; Peterson, U.S. Pat. No.3,502,538; Hartman, U.S. Pat. No. 3,502,763; Dobo et al., U.S. Pat. No.3,542,615; and Harmon, Canadian Patent Number 803,714 which are allincorporated herein by reference in their entirety.

The spunbond process also can be used to form bicomponent spunbondnonwoven webs as, for example, from side-by-sidepolyethylene/polypropylene spunbond bicomponent fibers. The process forforming such fibers and resultant webs includes using a pair ofextruders for separately supplying both the polyethylene and thepolypropylene to a bicomponent spinneret. Spinnerets for producingbicomponent fibers are well known in the art and thus are not describedherein in detail. In general, the spinneret includes a housingcontaining a spin pack which includes a plurality of plates having apattern of openings arranged to create flow paths for directing the highmelting temperature and low melting temperature polymers to eachfiber-forming opening in the spinneret. The spinneret has openingsarranged in one or more rows and the openings form a downwardlyextending curtain of fibers when the polymers are extruded through thespinneret. As the curtain of fibers exit the spinneret, they arecontacted by a quenching gas which at least partially quenches thefibers and develops a latent helical crimp in the extending fibers.Oftentimes the quenching air will be directed substantiallyperpendicularly to the length of the fibers at a velocity of from about100 to about 400 feet per minute at a temperature between about 45° andabout 90° F.

A fiber draw unit or aspirator is positioned below the quenching gas toreceive the quenched fibers. Fiber draw units or aspirators for use inmeltspinning polymers are well known in the art. Exemplary fiber drawunits suitable for use in the process include linear fiber aspirators ofthe type shown in U.S. Pat. No. 3,802,817 to Matsuki et al. and eductiveguns of the type shown in the U.S. Pat. Nos 3,692,618 to Dorshner et al.and 3,423,266 to Davies et al. all of which are incorporated herein byreference in their entirety. The fiber draw unit in general has anelongated passage through which the fibers are drawn by aspirating gas.The aspirating gas may be any gas, such as air that does not adverselyinteract with the polymers of the fibers. The aspirating gas can beheated as the aspirating gas draws the quenched fibers and heats thefibers to a temperature that is required to activate the latent crimpstherein. The temperature required to activate the latent crimping withinthe fibers will range from about 110° F. to a maximum of less than themelting point of the low melting component polymer which, in this case,is the polyethylene. Generally, a higher air temperature produces ahigher number of crimps per unit length of the fiber.

The drawn and crimped fibers are deposited onto a continuous formingsurface in a random manner, generally assisted by a vacuum device placedunderneath the forming surface. The purpose of the vacuum is toeliminate the undesirable scattering of the fibers and to guide thefibers onto the forming surface to form a uniform unbonded web ofbicomponent fibers. If desired, the resultant web can be lightlycompressed by a compression roller before the web is subjected to abonding process.

To bond the bicomponent spunbonded web a through-air bonder is used.Such through-air bonders are well known in the art and therefore neednot be described herein in detail. In the through-air bonder, a flow ofheated air is applied through the web to heat the web to a temperatureabove the melting point of the lower melting point component of thebicomponent fibers but below the melting point of the higher meltingpoint component. Upon heating, the lower melting polymer portion of theweb fibers are melted and the melted portions of the fibers adhere toadjacent fibers at the cross-over points while the high melting polymerportions of the fibers tend to maintain the physical and dimensionalintegrity of the web.

Bonded carded webs are made from staple fibers which are usuallypurchased in bales. The bales are placed in a picker which separates thefibers. Next, the fibers are sent through a combing or carding unitwhich further breaks apart and aligns the staple fibers in the machinedirection so as to form a machine direction-oriented fibrous nonwovenweb. Once the web has been formed, it is then bonded by one or more ofseveral bonding methods. One bonding method is powder bonding wherein apowdered adhesive is distributed through the web and then activated,usually by heating the web and adhesive with hot air. Another bondingmethod is pattern bonding wherein heated calendar rolls or ultrasonicbonding equipment are used to bond the fibers together, usually in alocalized bond pattern though the web can be bonded across its entiresurface if so desired. The best method though, when using bicomponentstaple fibers is to use a through-air bonder such as is described abovewith respect to the bicomponent spunbond web formation process.

In order to maintain the lofty and open nature of the resultant fibrousnonwoven web according to the present invention, the bonding processused to bond the fibers of the fibrous nonwoven web together should be aprocess such as through-air bonding which does not unduly compress orcollapse the structure during the formation process. In through-airbonding, heated air is forced through the web to melt and bond togetherthe fibers at their crossover points. Typically the unbonded web issupported on a forming wire or drum. In addition a vacuum may be pulledthrough the web if so desired to further contain the fibrous web duringthe bonding process.

Bonding processes such as point bonding and pattern bonding using smoothand/or pattern bonding rolls should generally be avoided if suchprocesses will create a resultant fibrous nonwoven web which is toodense and does not have the degree of voids necessary for the presentinvention. Whatever process is chosen, the degree of bonding will bedependent upon the fibers/polymers chosen but, in any event, it isdesirable that there be as little compression as possible during theheating stage.

Airlaying is another well known process by which fibrous nonwoven websaccording to the present invention can be made. In the airlayingprocess, bundles of small fibers usually having lengths ranging betweenabout 6 and about 19 millimeters are separated and entrained in an airsupply and then deposited onto a forming screen, oftentimes with theassistance of a vacuum supply. The randomly deposited fibers are thenbonded to one another using, for example, hot air or a spray adhesive.

In order to form a fibrous nonwoven web with the parameters outlinedbelow, at least a portion of the fibers forming the web must be madefrom polymers which are heat bondable. By heat bondable it is meant thatthe randomly deposited fibers forming the nonwoven web can be subjectedto heat or ultrasonic energy of a sufficient degree that the fibers willadhere to one another at the fiber cross over points due to the meltingor partial softening of the polymer forming the heat bondable fibers.Suitable polymers for forming such heat bondable fibers are permanentlyfusible and are typically referred to as being thermoplastic. Examplesof suitable thermoplastic polymers include, but are not limited to,polyolefins, polyesters, polyamides, orlon, acetates and polyvinylalcohol as well as homopolymers, copolymers and blends. In addition,wetting agents/surfactants may be added either internally, such as withsiloxane during the fiber formation process, or externally as a posttreatment either to the fibers and/or the resultant web as with anionicor nonionic surfactants including fluorocarbons. Such wettingagents/surfactants as well as their use are well known and need not bedescribed herein in detail.

The fibers formed from the aforementioned polymers may be short staplelength fibers such as are used in the airlaying and the bonding andcarding processes or longer more continuous fibers as are formed in, forexample, the spunbond process. Typical staple fiber lengths will rangebetween about 38 and about 51 millimeters, though lengths outside thisrange also may be used. For example, airlaying typically involves usingfibers with lengths in the range of about 6 to about 19 millimeters.Fiber diameters will be governed by the surface area to void volumeparameters set forth below. Typically the fiber diameters will rangebetween about 1.5 and about 16 denier with the target range beingbetween about 3 and about 6 denier.

In order to achieve the lofty open structure of the material of thepresent invention, it is desirable that the fibers be crimped. Crimpingcan be imparted both mechanically and chemically thereby forming bothzig zag or saw tooth and helically or spirally crimped fibers. Fibercross-sections may be either circular or noncircular including, forexample bilobal, trilobal, and X-shaped cross-sections. The fibers maybe solid or hollow. In addition they may be made from a single fiberpolymer or from multiple polymers such as are commonly found inbiconstitutent and bi- or multicomponent fibers. When using bicomponentfibers, fiber cross-sections may include, for example, sheath/core,side-by-side and islands-in-the-sea cross-sections.

The resultant fibrous nonwoven web will generally be a homogenous singlelayer blend of whatever type fiber or fibers are chosen. It is alsopossible, however, to form multilayer structures provided they meet theparameters set forth with respect to the present invention.

To facilitate the through-air bonding process, it has been foundadvantageous to use bicomponent fibers which have both a higher meltingpoint and lower melting point component such as in a side-by-side,sheath/core or islands-in-the-sea configurations. The lower meltingpoint component or polymer of the bicomponent fibers provides anefficient means for bonding the fibers together while the higher meltingpoint component aids in maintaining the structural rigidity and theopenness of the material both in the dry and wet states. Suitablebicomponent fibers include, for example, whether in staple fiber or morecontinuous spunbond form, polyethylene/polypropylene andpolyethylene/polyester fibers. The fibrous nonwoven web according to thepresent invention may be made entirely from bicomponent fibers or it maybe made from a blend of bicomponent fibers and other fibers such assingle component fibers including polyesters, nylons, rayons andpolyolefins such as polypropylene. It also may be made exclusively fromsingle component fibers. Generally, the fibrous nonwoven web accordingto the present invention will include at least 50% by weight bicomponentfibers, based upon the total weight of the web. Such bicomponent fiberswill typically have an average denier equal to or greater than 2.

The material of the present invention has been designed based upon aspecific set of parameters. These parameters include basis weight, voidvolume, permeability, porosity, surface area per void volume,hydrophilicity, compression resiliency and saturation capacity. Thebasis weight of the fibrous nonwoven web according to the presentinvention will be at least 20 grams per square meter with no real upperlimit and with the target range being from about 40 to about 68 gramsper square meter. The void volume of the fibrous nonwoven web is ameasure of how much air space there is in the structure. The void volumeas explained in the test procedures below is measured at 689 dynes persquare centimeter (0.01 pounds per square inch) and will range fromabout 80 to about 117 cubic centimeters per gram of web with the targetrange being from about 80 to about 100 cubic centimeters per gram ofweb. The permeability indicates the ability of the structure to conductfluid through it. When a fluid initially enters a surge structure, fluidmovement is dominated by forced flow from the momentum of the fluid.Capillarity may not be significant in this flow regime as it may nothave enough time to control the fluid path, thus, fluid flow through thestructure will be controlled by the permeability of the structure on theinitial insult. A high permeability value indicates that it isrelatively easy for a fluid to flow through the structure. Permeabilityfor the materials according to the present invention will range betweenabout 7.8×10⁻⁵ to about 1.5×10⁻⁴ square centimeters (8,000 to 15,000darcy). Outside this range other materials have been found not to workas well.

The porosity of the nonwoven web is the ratio of the amount of voidspace in a web to the total volume of the web. The porosity of thematerials according to the present invention, as measured at a pressureof 689 dynes per square centimeter (0.01 pounds per square inch), willrange from about 98.6% to about 99.4%.

The surface area per void volume with the void volume being measured at689 dynes per square meter (0.01 pounds per square inch) pressure willrange from about 10 to about 25 square centimeters per cubic centimeter.Permeability is the result of fluid having to travel over and aroundfiber surfaces when under forced flow in order to occupy the void spaceswithin the web. Surface area per void volume (SA/VV) indicates howclosely together those fiber surfaces are located to each other. Thus,SA/VV can control the amount of permeability for a structure. A highSA/VV value indicates there is a large amount of surface area which isplaced closely together. Increases in SA/VV can be achieved by usingsmaller fibers which increases the surface area per unit weight or bymaking the structure more dense which decreases the void volume per unitweight. When SA/VV is increased, permeability decreases since fluid isforced to travel over and around more surfaces to get through thestructure. If the SA/VV becomes too high, then the permeability will betoo low to allow easy fluid entry into and flow through the surgestructure. Thus, SA/VV must stay below 25 cm² /cc in order for thepermeability to be above about 8,000 darcy.

To ensure rapid intake of liquid, the overall structure must havehydrophilic tendencies. At least a portion of the fibers must have acontact angle less than 90 degrees. As a result, a fibrous nonwoven webaccording to the present invention will have sufficient hydrophilictendencies when the web has a saturation capacity greater than 55 gramsof 0.9% saline solution per gram of web. Another important feature ofthe material of the present invention is its resiliency in both the wetand dry states. A unique feature of the present invention is the amountof liquid which the material is able to absorb upon rapid insult. Inaddition, once the liquid has been absorbed, the material of the presentinvention does not readily collapse which would be detrimental to theoverall performance of the material in that the collapsing of thematerial would result in a reduced capacity for retaining liquid.Materials according to the present invention should have compressionresilience values in both the wet and dry states of at least about 60%.

In order to demonstrate the properties of the present invention, aseries of materials were formed and then tested relative to whether theymet the parameters outlined above. In addition, samples of thesematerials were then placed within diaper constructions and tested forleakage properties. The test procedures, materials and test results areset forth below.

VOID VOLUME AND COMPRESSION RECOVERY/RESILIENCY TEST

Void volume and compression recovery were measured using an INSTRON orSINTECH tensile tester to measures the resisting force as a material iscompressed between a movable platen and a fixed base at a constant rateusing a certain amount of force and subsequently releasing the force atthe same rate. Preferably pressure, or force, and platen position arerecorded. If only force is recorded, pressure is calculated using:##EQU1## where: P=pressure in Pascals

F=force pushing back on the platen in Newtons

A_(p) =area of the platen in square centimeters (18.9 cm²)

Void volume for a given platen position is calculated using theequation: ##EQU2## where: VV=void volume of material in cubiccentimeters per gram

x_(o) =initial platen position from the base in millimeters

x=platen position from initial position in millimeters

A_(m) =area of material in square centimeters

M=mass of material in grams

ρ_(fiber) =fiber density in grams per cubic centimeter

For webs made with multiple fiber types, the web fiber density is theweight average of each individual fiber density:

    ρ.sub.fiber,Total =wt %.sub.fiber 1 ·ρ.sub.fiber 1 +wt %.sub.fiber 2 ·ρ.sub.fiber 2 + . . .

where:

wt %=weight percent of the fiber type in the web or: ##EQU3##

The base must be larger in size than the platen. Zero height betweenplaten and base distance was set by bringing the platen down until itbarely touches the base. The platen was then raised to the desiredinitial height from the zero distance. The initial platen position mustbe greater than the initial thickness of the material so that the teststarts out at zero pressure on the sample. The material can be the samesize as the platen or larger.

Suitable equipment for this test could include:

Compression tester:

INSTRON model 6021 with compression test software and 1 kN load cellmade by Instron of Bucks, England.

Balance:

Mettler of Highstown, N.J., model PM4600

For the purpose of measuring void volume for the present specification,a 4.9 cm diameter circular platen was used to compress materials againstthe base at a rate of 5.08 mm/min up to 909 gm load (4,690 Pascal or0.68 pounds per square inch pressure). The platen was then returned atthe same rate to the initial starting position. The initial startingposition for the platen was 12.7 mm from the base. Material samples werecut to 10.2 cm×10.2 cm and tested in the center. Force and position datawere recorded every 0.03 minutes or every 0.15 mm. Five repeats wereperformed on separate material pieces.

Wet void volume was measured similarly to the dry void volume except thesample was completely immersed in 0.9% saline throughout the test. Aflat bottomed container such as a hexagonal polystyrene weighing dishcatalog #02-202D from Fischer Scientific of Pittsburgh, Pa. was placedon the base and the platen was zeroed and set to the initial position asdescribed in the dry void volume method. 0.9% saline was added to thecontainer to fill it to a level just to the bottom of the platen at itsinitial position. At this point in the procedure the load cell wastared. An appropriate saline could be S/P certified blood bank salinemade by Stephens Scientific of Riverdale, N.J. and distributed by BaxterHealthcare of McGraw Park, Ill. under catalog #B3158-1. The load cellwas tared with this level of fluid in the container. The sample wasplaced in the fluid, under the platen and the test was then performed asdescribed above for the dry void volume. Buoyant force was found to havea negligible affect on pressure but if so desired it can be subtractedfrom the pressure readings at each platen position using the followingequation: ##EQU4## where: P_(B) =Pressure from buoyant force in Pascals

ρ_(saline) =saline (fluid) density in grams per cubic centimeter

A_(p) =area of the platen in square centimeters (18.9 cm²)

A_(d) =area of the dish in square centimeters

x_(o) =initial platen position from the base in millimeters

x=platen position in millimeters

g=standard acceleration of gravity which is 981 centimeters per secondssquared

0.01=conversion factor=0.1 cm/mm·0.001 kg/gm·100 cm/m

The overall pressure on the sample becomes:

    P.sub.sample =P.sub.reading -P.sub.B

where:

P_(sample) =pressure on the sample from the platen in Pascal

P_(reading) =pressure reading from the SINTECH or INSTRON in Pascal

P_(B) =buoyancy pressure from the 0.9% saline in Pascal

For the purpose of measuring void volume for the present specifications,120 ml of saline was placed in the container and the platen wasinitially set 12.7 mm from the base.

Percent recovery at 68.9 Pascal (0.01 psi) was calculated using theplaten positions on compression and recovery when the pressure was 68.9Pascal: ##EQU5## where: VV_(recovery) 68.9 Pa =void volume upon recoveryat 68.9 Pascal pressure

VV_(compress) 68.9 Pa =void volume upon compression at 68.9 Pascalpressure

POROSITY CALCULATION

Porosity is the ratio of the amount of void space in a web to the totalvolume of that web, or:

    φ=(1-ρ.sub.web /ρ.sub.fiber)·100%

where:

φ=porosity of the web and is dimensionless

ρ_(web) =web density in grams per cubic centimeter

ρ_(fiber) =fiber density in grams per cubic centimeter

If a web is made of multiple fiber types with different densities, thenthe total fiber density is the weight average of the individual fiberdensities:

    ρ.sub.fiber, Total =wt %.sub.fiber 1 ·ρ.sub.fiber 1 +wt %.sub.fiber 2 ·ρ.sub.fiber 2 + . . .

where:

wt %=weight percent of the fiber type in the web

The void volume of the web can be used to calculate porosity instead ofthe web density: ##EQU6## where: VV=void volume of the web in cubiccentimeters per gram of web. Void volume is calculated in accordancewith the procedures set forth under the "Void Volume and CompressionRecovery/Resiliency Test" above.

As a material becomes more open in structure, the porosity will approachthe asymptote one, indicating empty space.

PERMEABILITY TEST

Permeability indicates the ease or difficulty with which a fluid flowsthrough a structure when a pressure gradient is applied to a fluid. Theresulting fluid velocity through the structure is controlled by thepermeability of the structure. The permeability of these samples in theZ-direction, which is through the thickness of the material, wasmeasured by a forced flow test which is described in detail in anarticle by Bernard Miller and David B Clark entitled, "Liquid TransportThrough Fabrics; Wetting and Steady-State Flow" published in TextileResearch Journal, pages 150 through 155, (March 1978). The foregoingarticle is incorporated herein by reference in its entirety.

To perform the test, a forced flow resistance monitor was built inaccordance with the instructions in the foregoing article. In the forcedflow test, the sample was held in a cylinder and fluid was pushedthrough the material at a constant velocity by a piston and the backpressure against the piston was recorded. The permeability wascalculated using Darcy's Law which describes fluid flow through a porousmedium according to the following equation: ##EQU7## where:v=superficial flow velocity or piston velocity in centimeters per minute

Q=volume flow rate in cubic centimeters per second

A=cross sectional area of the inner diameter of the tube (31.7 squarecentimeters)

k_(z) =material permeability constant

dp=pressure drop minus the average wall resistance pressure, both inPascals

z=thickness in centimeters of the material. Sum of all layers.

dp/dz=pressure gradient across the material in Pascals per centimeter

μ=fluid viscosity (cp) which is about 6 cp for Penetek oil

This equation can in turn be solved for permeability in the Z-direction(k_(z)) in the units of darcy as follows: ##EQU8##

The pressure drop for the test was obtained using a plot of the pressurevs. time. The pressure drop is equal to the change in pressure betweenthe pause point C in FIG. 1 of the drawings and when the piston startsup again at point D in FIG. 1. The wall resistance which was subtractedfrom the pressure drop was measured by running the test without a samplein the cylinder and measuring the same pressure drop. The measurement ofthe wall resistance was done five times and then averaged.

The thickness of the material can be obtained either by performing thecompression resiliency test described above at a maximum pressure of 689dynes per square centimeter (0.01 pounds per square inch) on eachsample, or measuring the sample's basis weight and using the averagespecific volume (inverse density) from the compression resiliency test.The thickness calculation for the latter method is: ##EQU9## where:M=mass of material in grams

A_(sample) =area of sample which was 45.6 square centimeters

V_(sp), 68.9 Pa =average specific volume for the material from thecompression resiliency test at 68.9 Pascals (0.01 pounds per squareinch) in cubic centimeters per gram of web

Since the standard deviation of the specific volume at 68.9 Pascal wasless than ten percent of the average, the assumption of constantspecific volume was considered to be valid. If the specific volumestandard deviation is above ten percent, then it must be measureddirectly using the compression resiliency test described above. When twoor more samples were layered, the samples were oriented with theirmachine directions from formation in the same direction and stacked ontop of each other top-to-bottom (i.e. the air side from formation to thewire side from formation).

The equipment used in conjunction with the test apparatus included apressure transducer Model #P3061-50WD from Lucas-Schaevitz Company ofPennsauken, N.J. This pressure transducer was capable of measuring up to50 inches of water (12,440 Pa) pressure. The additional equipment usedincluded a chart recorder Model SE 120, 881221100 from BBC GoerzMetrawatt of Austria; a slide and motor positioner model #B4036W1J fromVelmex, Inc. of Holcomb, N.Y.; a stepper motor controller model #14V 8KBASIC from Centroid Corporation of State College, Pa.; and, a COMPAQ®personal computer with a serial port.

Calibration of the pressure measurements was accomplished by adding aknown weight or volume of fluid to the cylinder and comparing thepressure transducer response to the theoretical pressure increase usingthe formula:

    ΔP.sub.theory =ρ·g·h=g·M/A·100 cm/m·0.001 kg/gm

where:

ΔP_(theory) =theoretical pressure change in Pascals

ρ=fluid density in grams per cubic centimeter

g=standard acceleration of gravity which is 981 centimeters per squaresecond

h=height of fluid added to cylinder in centimeters

A=inner area of cylinder in square centimeters which was 31.7 squarecentimeters

M=fluid mass in grams

The Pressure change from the transducer and strip chart recorder wascalculated using the equation: ##EQU10## where: ΔP_(trans) =pressuretransducer reading in Pascals

Chart_(read) =chart reading on x-y plotter in number of gradations

FS_(chart) =full scale (total gradations) on x-y plotter in number ofgradations

Chart_(volt) =chart recorder full scale voltage setting in millivolts

PT_(volt) =pressure transducer full scale voltage range in millivolts

PT_(pres) =pressure transducer full scale pressure range in inches ofwater

In deriving the permeability data, no screens were used to hold thesamples in place. Instead, two halves of a 6.35 cm inner diametercylinder which screwed together with a 7.62 cm diameter samplepositioned between the two cylinder pieces. Mineral oil was used as thefluid. Specifically, the mineral oil was Penetek technical grade mineraloil from Penreco of Los Angeles, Calif. The mineral oil had a viscosityof approximately 6 centipoise. The piston velocity was 20 cm/min. Theresults given in the data section were the average of the results offive separate tests on five separate samples of material and werereported in darcy.

SURFACE AREA PER VOID VOLUME CALCULATION (SA/VV)

Surface areas of fabrics composed of round cross-section staple fiberscan be calculated directly. The surface area is calculated from fiberdenier using the relationship: ##EQU11## where: SA=surface area insquare centimeters per gram of web or fabric

Denier=fiber size in denier which is the weight in grams of a singlefiber 9000 meters in length

ρ_(fiber) =fiber density in grams per cubic centimeter

3363=conversion factor from square root Denier divided by Denier tosquare root of diameter (in centimeters) divided by length (incentimeters)

If the surge material is made of a blend of fibers, then the totalsurface area per unit weight for the material is the weight average:

    SA.sub.total =wt %.sub.fiber 1 ·SA.sub.fiber 1 +wt %.sub.fiber 2 ·SA.sub.fiber 2 + . . .

where:

SA_(total) =total surface area in the web in cm² /g

wt %_(fiber) 1 =weight percent of the fiber in the web

SA_(fiber) 1 =surface area of the fiber in cm² /g

The fiber-surface-areas within webs composed of modified cross-sectionfibers, such as modified cross-section staple fibers, modifiedcross-section melt extruded fibers can be measured by the BET method ofBrunauer, Emmett and Teller, Journal of the American Chemical Society,60, 309 (1938) which is incorporated herein by reference in itsentirety.

The BET technique involves the absorption of a mono-molecular layer ofgas molecules on to the surface of the fibers. Calculations regardingthe amount of gas present on the fibers yields a quantification of thefiber surface area values. This method has been used fairly routinely inthe paper industry for fibrous webs, such as papers, fillers and filtermaterials.

SA/VV then is the calculated ratio of the fiber surface area per unitweight (square centimeter per gram) to the web's void volume per unitweight (cubic centimeters per gram). SA/VV thus has the units of squarecentimeters per cubic centimeters.

SATURATION CAPACITY TEST

The saturation capacity test measures the saturation capacity of thematerial while in the horizontal plane and under a no load condition.The no load horizontal saturation capacity test takes a small sample ofmaterial, places it in a liquid environment and measures the maximumamount of liquid the material will hold. The value obtained is a resultof the interaction between the void volume, pore size distribution andwettability of the material.

The equipment and materials include a pan measuring at least 15 cmwide×20 cm long×5 cm deep. The pan should not be greater than 10 cmdeep. A second pan measuring at least 12 cm wide×12 cm long×1.3 cm witha maximum depth not exceeding 10 cm. It is best if this pan is made froma lightweight material as it will be involved in the weighing process.Also needed is a balance readable to 0.01 grams and the balance shouldbe covered so that the air currents do not affect the readings. Lastly,a saline solution is needed such as 0.9 per cent S/P Certified BloodBank Saline Solution Catallog B3158-1 from Baxter HealthcareCorporation.

Sample preparations involved cutting samples of the materials to betested. The dimensions of the samples should be 10 cm×10 cm square. Thefirst pan measuring 15×20 cm×5 cm should be filled with the aboveidentified saline solution to within 1.3 cm from the top of the pan.

Each sample should be individually weighed on the balance and the weightrecorded as the dry weight. Next, the tare weight of the dry 12 cm×12cm×1.3 cm pan should be measured and recorded. The tare 12 cm×12 cm×1.3cm pan is placed next to the 15 cm×20 cm×5 cm pan. Care should be takento wipe up any spilled liquid on the surface upon which the tared pan isplaced as this will affect the tare weight.

A sample is placed into the pan containing the saline solution and isallowed to remain in the solution undisturbed until it becomes submergedor for a maximum of 15 seconds, whichever occurs first. If the samplefloats on top of the solution it should not be forced down into thesolution as a floating sample is an indication of an insufficientlyhydrophilic material. Once the sample has completely submerged or the 15second period has elapsed, the sample should be withdrawn from thesaline solution by using both hands and grabbing the sample at all fourcorners using the middle and index finger to grab one corner of one sideand the thumb and ring finger for grabbing the other corner on the sameside. The sample should be lifted from the solution and placed in thetared pan as quickly as possible. Next, the wet sample should beweighed, the tare weight subtracted and the wet weight recorded. Theabove procedure is repeated until all samples are tested. As a specialnote, most of the samples being tested will contain a surfactant whichwill wash out into the saline solution. As a result, the solution shouldbe changed after approximately 40 samples have been wetted or when thelevel in the pan drops to about 2.5 cm.

To calculate the saturation capacity, the wet weight for each sample isdivided by the dry weight and the number one is then subtracted from theresult to eliminate the contribution of the dry weight in the numerator.The resultant number is the saturation capacity of the material which isrecorded as grams of liquid per grams of sample.

EXAMPLES

Having described the test procedures for evaluating the parameters ofthe materials according to the present invention, a series of samplematerials were prepared and tested for their properties. A summary ofthese materials and their properties is set forth below. Six samplematerials were prepared including both single-layer and two-layerfibrous nonwoven webs. The target basis weight for Examples 1 and 3through 6 was approximately 50 grams per square meter. The actual basisweights are given in Table 1. The target basis weight for Example 2 was80 grams per square meter with the actual value being 83.4 grams persquare meter (gsm). In examples 3 and 4, a two-layer structure wascreated with the first layer being approximately 13 grams per squaremeter and the second layer being approximately 37 grams per squaremeter. All samples were prepared using conventional carding equipmentand were subsequently through-air bonded at temperatures and timessufficient to cause the lower melting point component of the bicomponentfibers to at least partially melt and bond to one another at theircrossover points.

In Example 1, a 50.5 gsm fibrous nonwoven web was created using ahomogeneous blend of 60 percent by weight 4.4 denier by 38 mm millimetersoft 71 polypropylene fibers from Danaklon a/s of Denmark and 40 weightpercent 3.0 denier by 38 millimeter polyethylene sheath/polyester corebicomponent fibers from BASF Corporation Fibers Division of Enka, N.C.Both these fibers, as with all fibers contained within the examples,contained a manufacturer's finish which made the fibers hydrophilic. Allfibers were targeted by the manufacturer to have contact angles lessthan 90°. The two fibers were uniformly mixed together, carded and thenbonded using hot air at a temperature of approximately 143° C. for 4seconds to bond the overall structure. The void volume, surface area pervoid volume, porosity, permeability and saturation capacity values forthis material are set forth in Table 1.

In Example 2, an 83.4 gsm single-layer fibrous nonwoven web was madeusing 20 percent by weight 6.0 denier by 51 millimeter Hoechst Celanesetype 293 polyester staple fibers from Hoechst Celanese Textile FibersGroup of Charlotte, N.C.; 60 percent by weight of thepreviously-described 3.0 denier BASF bicomponent fibers and 20 percentby weight 1.5 denier by 38 millimeter Hoechst Celanese type 183polyester staple fibers from Hoechst Celanese Textile Fibers Group ofCharlotte N.C. As with Example 1, these fibers were uniformly mixedtogether, carded and then bonded using hot air at a temperature ofapproximately 135° C. for approximately 4 seconds. The properties ofthis material are set forth in Table 1.

In Example 3, a two-layer structure was created including a first layerof the previously described three-denier BASF polyethylenesheath/polyester core bicomponent fibers having a basis weight ofapproximately 13 gsm. Next, a second layer was formed on top of thefirst layer and the two layers were then bonded together using hot airat approximately 129° C. for approximately 4 seconds. The second layercontained a uniform mixture of 20 percent by weight of thepreviously-described Hoechst Celanese type 293 six-denier polyesterfibers; 35 percent by weight of the BASF three-denier polyethylenesheath/polyester core bicomponent fibers; 40 percent by weight of theHoechst Celanese type 183 1.5-denier polyester fibers and 5 percent byweight of 2.0 denier by 38 millimeter polyethylene sheath/polypropylenecore ES-HB bicomponent fibers from Chisso Corporation of Osaka, Japan.The combined layers had a basis weight of 46.1 grams per square meterand the parameters for this material are set forth in Table 1.

In Example 4, a second two-layer structure was created. This two-layerstructure most closely resembles the current material being used in thecommercially-available diapers of the assignee of record. The top layerwas approximately 17 grams per square meter and contained thethree-denier BASF polyethylene sheath/polyester core bicomponent fibers.The second or bottom layer contained a homogeneous mix of 60 percent byweight of the Hoechst Celanese 6.0 denier type 295 polyester fibers; 35percent by weight of a 1.7 denier by 38 millimeter polyethylenesheath/polypropylene core type ES bicomponent fiber from ChissoCorporation of Osaka, Japan and 5 percent by weight of the 2.0 denierChisso-type ES-HB polyethylene sheath/polypropylene core bicomponentfibers. The two layers were bonded together in the same manner and usingthe same conditions as the material described in Example 3, and theresultant material had a basis weight of 46.1 grams per square meter.The test results are set forth in Table 1.

In Example 5, a single layer fibrous nonwoven web having a basis weightof 49.8 gsm was created using a uniform mixture of 40 percent by weightHoechst Celanese type 224 6.0-denier polyester staple fibers and 60percent by weight of a Chisso-type ES P 3.0-denier by 38 millimeterpolypropylene sheath/polypropylene core bicomponent fiber. The web wasbonded using hot air at a temperature of 135° C. for approximately 4seconds. The test results for this material are set forth in Table 1.

In Example 6, a homogeneous single-layer fibrous nonwoven web having abasis weight of 51.9 gsm was created using 20 percent by weight of theHoechst Celanese type 295 6.0-denier polyester fibers; 20 percent byweight of the Hoechst Celanese type 183 1.5 denier polyester fibers and60 percent by weight of the BASF 3.0-denier polyethylenesheath/polyester core bicomponent fibers. The homogeneous blend offibers was bonded together using hot air at a temperature of 135° C. forapproximately 4 seconds. The test results for this material are setforth in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                   COMP RES                              BASIS WT.                                                                            VOID VOL.                                                                            SA/VV                                                                              POROSITY                                                                             PERMEABILI                                                                            SAT CAP                                                                             %                              EXAMPLE                                                                              gsm    cc/g   cm.sup.2 /cc                                                                       %      DARCY   g/g   dry                                                                              wet                         __________________________________________________________________________    1      50.5   32     54.2 97.0   2569    21    93 89                          2      83.4   60     29.6 98.7   5528    44    75 76                          3      46.1   68     29.7 98.9   5565    46    71 75                          4      46.1   60     19.4 98.6   6647    49    73 78                          5      49.8   84     20.0 98.9   9256    59    76 76                          6      51.9   110    16.2 99.3   13,189  79    70 73                          __________________________________________________________________________

The fibrous nonwoven webs according to the present invention must have abasis weight of at least 20 grams per square meter, a void volumebetween about 80 and about 117 cubic centimeters per gram of web at apressure of 689 dynes per square meter (0.01 lbs. psi), a permeabilityof about 8,000 to about 15,000 darcy, a porosity of about 98.6% to about99.4% and a surface area per void volume of about 10 to about 25 squarecentimeter per cubic centimeters. Referring to Table 1, it can been seenthat the material defined in Example 1 had surface area per void volume,porosity and permeability values which were all outside this range. Withrespect to Example 2, it can be seen that only the conditions forporosity were met. For Example 3, the same was also true. Sample 4 camecloser to meeting the parameters of the present invention but stillfailed to meet the void volume and saturation capacity and permeabilityparameters with respect to the present invention.

Example 5 demonstrates all the properties of the present invention. Thesame was true with respect to Example 6. In addition, the materials ofExamples 5 and 6 both had sufficient degrees of both wet and drycompression resiliency. The wet and dry resiliency values for Example 5were respectively 76% and 76% while the wet and dry compressionresiliency values for Example 6 were 73% and 70%. Both of thesematerials therefore met the requirement that the compression resiliencein either the wet or dry state be at least 60%.

Example 5 was put into HUGGIES® Ultratrim Step 4 diapers as a surgematerial between the body side liner and the absorbent core. The surgematerial was lightly glued to both the liner and absorbent core usingNational Starch 34-5563 spray from the National Starch Company ofBridewater, N.J. The add-on level for the adhesive between the liner andthe surge material according to the present invention was approximately0.01 to 0.04 grams per diaper while the add-on level between the surgelayer and the tissue-wrapped absorbent core was approximately 0.03 to0.05 grams per diaper. These diapers were use tested and were found tohave a 6.4% overall leakage occurrance for males and a 3.7% overallleakage occurrance for females. These overall leakage numbers areconsidered excellent given the current state of the art in disposablediaper leakage protection.

Having thus described the invention in detail, it should be apparentthat various other modifications and changes can be made in the presentinvention without departing from the spirit and scope of the followingclaims.

We claim:
 1. A fibrous nonwoven web comprising:a plurality ofthermoplastic fibers heat bonded to one another to form a lofty nonwovenweb having a basis weight of at least 20 grams per square meter, a voidvolume of between about 80 and about 117 cubic centimeters per gram ofweb at 689 dynes per square centimeter pressure, a permeability of about8,000 to about 15,000 darcy, a porosity of about 98.6% to about 99.4%and a surface area per void volume of about 10 to about 25 squarecentimeters per cubic centimeter.
 2. The fibrous nonwoven web of claim 1wherein said basis weight ranges from about 40 to about 68 grams persquare meter.
 3. The fibrous nonwoven web of claim 1 wherein said webhas a saturation capacity between about 55 and about 80 grams of 0.9percent saline solution per gram of web.
 4. The fibrous nonwoven web ofclaim 3 wherein said web has a compression resilience of at least about60 percent.
 5. The fibrous nonwoven web of claim 1 wherein said webincludes at least 50% by weight bicomponent fibers, based upon the totalweight of said web.
 6. The fibrous nonwoven web of claim 5 wherein atleast a portion of said bicomponent fibers have a denier equal to orgreater than
 2. 7. The fibrous nonwoven web of claim 5 wherein said webfurther includes polyester fibers.
 8. The fibrous nonwoven web of claim5 wherein said bicomponent fibers are crimped.
 9. A personal careabsorbent article comprising:a body side liner and a backing layer withan absorbent core disposed therebetween, said article further includinga surge layer disposed between said body side liner and said absorbentcore, said surge layer comprising a plurality of thermoplastic fibersheat bonded to one another to form a lofty nonwoven web having a basisweight of at least 20 grams per square meter, a void volume of betweenabout 80 and about 117 cubic centimeters per gram of web at 689 dynesper square centimeter pressure, a permeability of about 8,000 to about15,000 darcy, a porosity of about 98.6% to about 99.4% and a surfacearea per void volume of about 10 to about 25 square centimeters percubic centimeter.
 10. The personal care absorbent article of claim 9wherein said basis weight ranges from about 40 to about 68 grams persquare meter.
 11. The personal care absorbent article of claim 9 whereinsaid web has a saturation capacity between about 55 and about 80 gramsof 0.9 percent saline solution per gram of web.
 12. The personal careabsorbent article of claim 11 wherein said web has a compressionresilience of at least about 60 percent.
 13. The personal care absorbentarticle of claim 9 wherein said web includes at least a 50% by weightbicomponent fibers, based upon the total weight of said web.
 14. Thepersonal care absorbent article of claim 13 wherein at least a portionof said bicomponent fibers have a denier equal to or greater than
 2. 15.The personal care absorbent article of claim 13 wherein said web furtherincludes polyester fibers.
 16. The personal care absorbent article ofclaim 13 wherein said bicomponent fibers are crimped.